The following quote comes from John Murray's book Principles of Conduct. While John Murray can occasionally make the easiest things difficult to understand, at the same time he always manages to come up with some brilliant insights on the subject he’s discussing. In the following quote he’s discussing personal discord and the Sixth commandment:
"The sixth commandment is but one concrete way of expressing the principle that human life, in all its aspects and in all its relationships, must be guarded and promoted. Have we sufficiently appreciated the fact that, in a sinless world, there would have been no 'against'? The essence of sin is comprehended in the word 'against'. Sin is first of all against God and because we are against God we are against our fellowman. It is an eloquent witness to this fact that, after the first sin of our first parents, the first overt sin in the realm of ethics that is brought to our attention in the Scripture is the sin of Cain in slaying his brother Abel. 'Cain rose up against Abel his brother, and slew him' (Genesis 4: 8). The first sin of our first parents was against God; the sin of Cain was 'against' his brother. It is this 'against' that the sixth commandment condemns and its positive counterpart is that we 'take all lawful endeavours to preserve our own life and the life of others'. The opposite of 'against' is concord, harmony, peace, and love. And the demand of love is no less than that we love our enemies (cf. Matthew 5: 44). We are to love those who are 'against' us. The 'against' on one side does not abrogate the requirement of love on the other; one 'against' does not justify another. It is nothing less than this that Jesus' interpretation and application of the sixth commandment exemplify. Could our Lord's ethic of human relations, summed up in the words, 'Thou shalt love thy neighbour as thyself, have come to more concrete and relevant expression than in his teaching here respecting the sixth commandment? The principle that undergirds the sixth commandment is the sanctity of life. Our Lord shows the endless ramifications of that principle and pushes his analysis to the source and fountain of its preservation and violation. The spring of its preservation is the agreement of love; the root of its violation is the rudimentary feeling of unholy enmity, the disruptive imagination of the thought of the heart whereby the concord of human relations is desecrated. The teaching of our Lord is to the effect that the sixth commandment brings within its purview tin enmity of the heart and all unnecessary and unholy dispute which fans the embers of animosity."